Acids and Bases - Real-life applications

Photo by: chagin

pHand Acid-Base Indicators

Though chemists apply the sophisticated structural definitions for
acids and bases that we have discussed, there are also more
"hands-on" methods for identifying a particular
substance (including complex mixtures) as an acid or base. Many of
these make use of the pH scale, developed by Danish chemist SØren
SØrensen (1868-1939) in 1909.

The term pH stands for "potential of hydrogen," and the
pH scale is a means of determining the acidity or alkalinity of a
substance. (Though, as noted, the term "alkali" has been
replaced by "base," alkalinity is still used as an
adjectival term to indicate the degree to which a substance displays
the properties of a base.) There are theoretically no limits to the
range of the pH scale, but figures for acidity and alkalinity are
usually given with numerical values between 0 and 14.

THE MEANING OF pH VALUES.

A rating of 0 on the pH scale indicates a substance that is virtually
pure acid, while a 14 rating represents a nearly pure base. A rating
of 7 indicates a neutral substance. The pH scale is logarithmic, or
exponential, meaning that the numbers represent exponents, and thus an
increased value of 1 represents not a simple arithmetic addition of 1,
but an increase of 1 power. This, however, needs a little further
explanation.

The pH scale is actually based on negative logarithms for the values
of H
3
O
+
(the hydronium ion) or H
+
(protons) in a given substance. The formula is thus pH =
−log[H
3
O
+
] or −log[H
+
], and the presence of hydronium ions or protons is measured according
to their concentration of moles per liter of solution.

pH VALUES OF VARIOUS SUBSTANCES.

The pH of a virtually pure acid, such as the sulfuric acid in car
batteries, is 0, and this represents 1 mole (mol) of hydronium per
liter (l) of solution. Lemon juice has a pH of 2, equal to 10
−2
mol/l. Note that the pH value of 2 translates to an exponent of
−2, which, in this case, results in a figure of 0.01 mol/l.

Distilled water, a neutral substance with a pH of 7, has a hydronium
equivalent of 10
−7
mol/l. It is interesting to observe that most of the fluids in the
human body have pH values in the neutral range blood (venous, 7.35;
arterial, 7.45); urine (6.0—note the higher presence of acid);
and saliva (6.0 to 7.4).

At the alkaline end of the scale is borax, with a pH of 9, while
household ammonia has a pH value of 11, or 10
−11
mol/l. Sodium hydroxide, or lye, an extremely alkaline chemical with
a pH of 14, has a value equal to 10
−14
moles of hydronium per liter of solution.

LITMUS PAPER AND OTHER INDICATORS.

The most precise pH measurements are made with electronic pH meters,
which can provide figures accurate to 0.001 pH. However, simpler
materials are also used. Best known among these is litmus paper (made
from an extract of two lichen species), which turns blue in the
presence of bases and red in the presence of acids. The term
"litmus test" has become part of everyday language,
referring to a make-or-break issue—for example, "views
on abortion rights became a litmus test for Supreme Court
nominees."

Litmus is just one of many materials used for making pH paper, but in
each case, the change of color is the result of the neutralization of
the substance on the paper. For instance, paper coated with
phenolphthalein changes from colorless to pink in a pH range from 8.2
to 10, so it is useful for testing materials believed to be moderately
alkaline. Extracts from various fruits and vegetables, including red
cabbages, red onions, and others, are also applied as indicators.

Some Common Acids and Bases

The tables below list a few well-known acids and bases, along with
their formulas and a few applications

Common Acids

Acetic acid (CH
3
COOH): vinegar, acetate

Acetylsalicylic acid (HOOCC
6
H
4
OOCCH
3
): aspirin

Ascorbic acid (H
2
C
6
H
6
O
6
): vitamin C

Carbonic acid (H
2
CO
3
): soft drinks, seltzer water

Citric acid (C
6
H
8
O
7
): citrus fruits, artificial flavorings

Hydrochloric acid (HCl): stomach acid

Nitric acid (HNO
3
): fertilizer, explosives

Sulfuric acid (H
2
SO
4
): car batteries

Common Bases

Aluminum hydroxide (Al[OH]
3
): antacids, deodorants

Ammonium hydroxide (NH
4
OH): glass cleaner

Calcium hydroxide (Ca[OH]
2
): caustic lime, mortar, plaster

Magnesium hydroxide (Mg[OH]
2
): laxatives, antacids

Sodium bicarbonate/sodium hydrogen carbonate (NaHCO
3
): baking soda

Sodium carbonate (Na
2
CO
3
): dish detergent

Sodium hydroxide (NaOH): lye, oven and drain cleaner

Sodium hypochlorite (NaClO): bleach

Of course these represent only a few of the many acids and bases that
exist. Selected substances listed above are discussed briefly below.

Acids

ACIDS IN THE HUMAN BODY AND FOODS.

As its name suggests, citric acid is found in citrus
fruits—particularly lemons, limes, and grapefruits. It is also
used as a flavoring agent, preservative, and cleaning agent. Produced
commercially from the fermentation of sugar by several species of
mold, citric acid creates a taste that is both tart and sweet. The
tartness, of course, is a function of its acidity, or a manifestation
of the fact that it produces hydrogen ions. The sweetness is a more
complex biochemical issue relating to the ways that citric acid
molecules fit into the tongue's "sweet"
receptors.

Citric acid plays a role in one famous stomach remedy, or antacid.
This in itself is interesting, since antacids are more generally
associated with alkaline substances, used for their ability to
neutralize stomach acid. The fizz in Alka-Seltzer, however, comes from
the reaction of citric acids (which also provide a more pleasant
taste) with sodium bicarbonate or baking soda, a base. This reaction
produces carbon dioxide gas. As a preservative, citric acid prevents
metal ions from reacting with, and thus hastening the degradation of,
fats in foods. It is also used in the production of hair rinses and
low-pH shampoos and toothpastes.

The carboxylic acid family of hydrocarbon derivatives includes a wide
array of substances—not only citric acids, but amino acids.
Amino acids combine to make up proteins, one of the principal
components in human muscles, skin, and hair. Carboxylic acids are also
applied industrially, particularly in the use of fatty acids for
making soaps, detergents, and shampoos.

SULFURIC ACID.

There are plenty of acids found in the human body, including
hydrochloric acid or stomach acid—which, in large quantities,
causes indigestion, and the need for neutralization with a base.
Nature also produces acids that are toxic to humans, such as sulfuric
acid.

Though direct exposure to sulfuric acid is extremely dangerous, the
substance has numerous applications. Not only is it used in car
batteries, but sulfuric acid is also a significant component in the
production of fertilizers. On the other hand, sulfuric acid is
damaging to the environment when it appears in the form of acid rain.
Among the impurities in coal is sulfur, and this results in the
production of sulfur dioxide and sulfur trioxide when the coal is
burned. Sulfur trioxide reacts with water in the air, creating
sulfuric acid and thus acid rain, which can endanger plant and animal
life, as well as corrode metals and building materials.

Bases

The alkali metal and alkaline earth metal families of elements are, as
their name suggests, bases. A number of substances created by the
reaction of these metals with nonmetallic elements are taken
internally for the purpose of settling gastric trouble or clearing
intestinal blockage. For instance, there is magnesium sulfate, better
known as Epsom salts, which provide a powerful laxative also used for
ridding the body of poisons.

Aluminum hydroxide is an interesting base, because it has a wide
number of applications, including its use in antacids. As such, it
reacts with and neutralizes stomach acid, and for that reason is found
in commercial antacids such as Di-Gel™, Gelusil™, and
Maalox™. Aluminum hydroxide is also used in water purification,
in dyeing garments, and in the production of certain kinds of glass. A
close relative, aluminum hydroxychloride or Al
2
(OH)
5
Cl, appears in many commercial antiperspirants, and helps to close
pores, thus stopping the flow of perspiration.

SODIUM HYDROGEN CARBONATE (BAKING SODA).

Baking soda, known by chemists both as sodium bicarbonate and sodium
hydrogen carbonate, is another example of a base with multiple
purposes. As noted earlier, it is used in Alka-Seltzer™, with
the addition of citric acid to improve the flavor; in fact, baking
soda alone can perform the function of an antacid, but the taste is
rather unpleasant.

Baking soda is also used in fighting fires, because at high
temperatures it turns into carbon dioxide, which smothers flames by
obstructing the flow of oxygen to the fire. Of course, baking soda is
also used in baking, when it is combined with a weak acid to make
baking powder. The reaction of the acid and the baking soda produces
carbon dioxide, which causes dough and batters to rise. In a
refrigerator or cabinet, baking soda can absorb unpleasant odors, and
additionally, it can be applied as a cleaning product.

SODIUM HYDROXIDE (LYE).

Another base used for cleaning is sodium hydroxide, known commonly as
lye or caustic soda. Unlike baking soda, however, it is not to be
taken internally, because it is highly damaging to human
tissue—particularly the eyes. Lye appears in drain cleaners,
such as Drano™, and oven cleaners, such as Easy-Off™,
which make use of its ability to convert fats to water-soluble soap.

In the process of doing so, however, relatively large amounts of lye
may generate enough heat to boil the water in a drain, causing the
water to shoot upward. For this reason, it is not advisable to stand
near a drain being treated with lye. In a closed oven, this is not a
danger, of course; and after the cleaning process is complete, the
converted fats (now in the form of soap) can be dissolved and wiped
off with a sponge.